Year-end production start expected for Cerro Colorado

Initial production from the Cerro Colorado porphyry copper deposit in northern Chile is expected to begin by year-end, Rio Algom (TSE) reports.

Over an estimated 23-year life, 10,000 tonnes per day will be mined. The result: 40,000 tonnes of electrolytic copper per year. Cash costs in 1992 dollars are expected to be US45-55 cents per lb. of cathode. The deposit, in the foothills of the Andean Cordillera, was first explored in 1963 by an American-Chilean group searching for a “blind” porphyry deposit beneath younger cover rocks adjacent to several copper oxide showings. Drilling identified a porphyry system with some supergene (secondary) copper sulphide mineralization. One of four holes drilled intersected 1.23% copper over 29 metres at a depth of 190 metres.

In the mid-1970s, Nippon Mining of Japan drilled 24 holes on a 200×200-metre grid pattern and carried out 1,350 metres of underground drifting. This work delineated the central portion of the orebody.

Rio and a partner began exploring the deposit in 1981. Early attempts to bring it into production failed because of low grades and poor copper prices. Yet another drawback was the volatile political climate in Chile during the early to mid-1980s.

Between 1981 and 1992, Rio drilled 97 holes to test the surrounding area and complete a 100×100-metre grid pattern to a depth of 200 metres. This drilling served to delineate 79 million tonnes grading .39% copper (70% sulphide ore and 30% oxide ore).

The deposit lies within a sequence of Cretaceous, andesitic, volcanic rocks consisting of porphyritic flows, breccias, agglomerates and lapilli tuffs. The volcanics are intruded by a series of late Cretaceous to mid-Tertiary tonalites and granodiorites typical of calc-alkaline porphyry copper-molybdenum systems.

Four types of alteration related to the intrusive porphyry system have been recognized: propyllitic, phyllic, potassic and silicic. The alteration has affected both the intrusives and the volcanics and, where intensely developed, has destroyed all primary textures.

The Cerro Colorado orebody is a subhorizontal, somewhat irregular, blanket-shaped zone of supergene mineralization developed over a larger and much lower-grade zone of primary mineralization. The supergene copper sulphide/oxide blanket is about 850 by 700 metres wide and 50 to 200 metres thick. It lies beneath 60 to 120 metres of barren leach capping and later cover rocks.

In the supergene sulphide zone, chalcocite and minor covellite occur as coatings and replacements of hypogene (primary) pyrite, chalcopyrite and bornite. Sulfide enrichment occurs primarily as disseminations in the andesitic volcanics and typically grades 1.5% copper. Mineralization in the intrusives is primarily in veins and averages about 1% copper. The supergene oxide zone contains mainly chrysocolla, lesser atacamite and rare malachite (all copper oxide minerals). Oxidation is prevalent along the lateral and upper fringes of the supergene sulphide zone. The grade of mineralization in both the volcanics and intrusives averages 1.3% copper. Primary sulphide mineralization consists predominantly of pyrite with minor chalcopyrite, bornite and molybdenite. These minerals occur with quartz veinlets and as disseminations in both the intrusives and volcanics. The average grade of the primary mineralization ranges from 0.2% to 0.5% copper and up to 0.025% molybdenum.

Cerro Colorado, unlike many other porphyry producers, will not be subjected to conventional milling and smelting. Instead, ore will be heap-leached and the copper recovered by solvent extraction (SE) and electrowinning(EW). Leaching will be carried out on special pads 770 metres long and 520 metres wide. The ore, piled six metres high, will be sprinkled with a weak acid solution for 210 days. The copper is leached with bacteria which form naturally and which help oxidize the sulphides. From the pads, the solution is pumped to the SE facilities.

In the SE process, copper is removed from solution by means of an organic extractant. It is then transferred into a solution rich in sulphuric acid and copper sulphate. This electrolyte solution is then pumped to the EW facilities where it is plated on to cathodes.

Some cathode will be produced by year-end and full production will be achieved by mid-1994, Rio said. It has already pre-sold 14 years of finished cathode copper to Norddeutsche Affinerie in Germany and Mitsubishi Corp. of Japan. The sales are unhedged as the price will be based on London Metal Exchange prices at the time of delivery.

According to Forbes West, Rio’s vice-president of human resources and communication, total capital costs for the project are estimated at US$290 million and expenditures to date total about US$198 million.